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Case Fans: Decoupling Done Right

How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 2

A Few Notes about Structure-Borne Sound

Most typically, sound levels are cited on data sheets in db(A). This means the sound level is adjusted by a curve that reflects the sound response of the human ear, the A-weighting curve. But this value only encompasses sound emitted through the air. Alas, it becomes almost meaningless for assessing the sound level of a fan mounted in a case.

In addition to emitted sound, there is also structure-borne sound to consider. What is structure-borne sound? It consists mostly of low-frequency vibrations that emanate from the motor and bearing, and are conducted from the fan to the case via the fan mount. A large chassis surface then dissipates these vibrations into the air like a loudspeaker membrane. Stiff, high-quality cases bleed out less sound from this phenomenon than flimsy ones. In addition to the loudspeaker effect, an enclosure's interior can act as a resonance chamber, aggravating the effect.

Correctly Decoupling Case Fans

Here’s the bad news: decoupling frames only benefit one entity, the vendor who sells them, since their mounting screws conduct most of the sound anyway. Rubber/silicone bolts or screws are much better because they barely conduct sound. Used together with a thin, flat gasket from the plumbing aisle of Home Depot, you get a perfectly decoupled fan. Be sure to use soft bolts instead of regular fan screws.

Fans with a Soft Rubber Frame

Companies like Xilence, Deepcool, and Noiseblocker offer case fans with a fan frame made of rubber, which can be screwed to a case without further ado. The whole frame decouples the fan noise, rendering rubber bolts unnecessary. A modern example is the 2CF from Xilence:

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